Ideally, a tire is desirable to be a perfect circle, and interior stiffness, dimensions and weight distribution and other features thereof should be uniform around the circumference of the tire. However, the usual tire construction and manufacturing process make it difficult to mass produce such an ideal tire. That is, a certain amount of nonuniformity in the stiffness, dimensions and weight distribution and other features occur in the produced tire. As a result, an exciting force is produced in the tire while the vehicle is running. The oscillations produced by this exciting force are transmitted to the vehicle chassis and cause a variety of vehicle oscillations and noises including shaking, fluttering, sounds of the tire vibrations being transmitted inside the vehicle, and beat sounds.
One known method for evaluating nonuniformity of a tire is described in Automobile Standards “Uniformity testing methods for automobile tires” (JASO C607). In this method, a rotating drum, which serves as a substitute for the road surface, presses against a rotatably held tire with a predetermined pressing force (several hundred kilograms), or the tire is pressed against the rotating drum with the predetermined pressing force. The tire and the rotating drum are capable of rotating around their respective rotational axes, in such a way that when either one rotates, the other is also caused to rotate.
In this condition, the tire or the rotating drum is rotatably driven so that the tire rotates at 60 [rpm]. As the tire rotates, the exciting force produced by nonuniformity of the tire occurs. This exciting force is measured by one or more means for measuring force (such as a load cell) mounted on a bearing which rotatably supports the tire or the rotating drum, or mounted on a member attached to this bearing. From the measured value, an index that serves to evaluate the nonuniformity of the tire is computed. This measurement is called as a uniformity measurement. The index obtained by means of this uniformity measurement is computed by modeling the tire as a disc (this model will be called as a “disc model” hereinafter) and assuming that the force is concentrated at the center of that disc.
Next, tires on which measurements were performed are classified into those for which the nonuniformity obtained from the index is within tolerable limits and those for which it is not. To the extent possible, tires for which the nonuniformity is outside of the tolerable limits are subjected to processing to decrease the nonuniformity. Tires that have been processed are then subjected to uniformity measurement again; those for which the nonuniformity is within tolerable limits are separated from those for which it is not.
Through the procedure described above, only tires judged to have “nonuniformity within tolerable limits” are selected and shipped to customers (or sent to the next step in the tire evaluation procedure).
Recently, a problem has occurred in that even when nonuniformity as measured by the uniformity measurement method described above is judged to be within tolerable limits, particularly in high speed vehicle operation, an exciting force from a tire is sometimes applied to the vehicle shaft, causing vehicle oscillations and noise inside the vehicle. The cause of these oscillations and noise is considered to be nonuniformity that could not be evaluated from the result of measurement by the conventional uniformity measurement method. Therefore, a nonuniformity measurement method that makes it possible to evaluate the nonuniformity that is causing these phenomena has been desired.